1. Field of the Invention
The present invention relates generally to top surface imaging processes, and more particularly to residue free patterning in to surface imaging resists.
2. Discussion of the Related Art
Fabrication of very large scale integrated circuits (VLSI) and ultra large scale integrated circuits (ULSI) requires that the resist materials, lithographic processes, and exposure tools meet necessary performance demands for high throughput manufacturing of sub-micron feature size devices (i.e., devices with feature sizes less than 1.0 .mu.m). In the instance of sub-micron lithography, top surface imaging is used to increase the resolution capability of optical exposure systems. Several TSI processes have been developed, most notably, Roland and Coopman's Diffusion Enhanced Silylated Resist (DESIRE) negative tone process, as discussed in F. Coopmans and B. Roland, Proc. of SPIE, 631 (1986), 34-39; B. Roland, R. Lombaerts, C. Jakus, and F. Coopmans, SPIE, Proc. of SPIE, 771 (1987), 69-76; and Roland, Microelectronic Eng., 13 (1991), 11-18. Another TSI process is the positive tone Silylated Acid Hardened Resist (SAHR) developed by Thackeray et al. as discussed in J.W. Thackeray, J F. Bohland, E.K. Pavelchek, G.W. Orsula, A.W. McCullough, S.K. Jones, S.M. Bobbio, Proc. of SPIE, 1185 (1989), 2-11.
Top surface imaging in general uses reactive ion etching (RIE) to dry develop patterns after exposure and silylation of a photoresist layer. A dry development process for top surface imaging requires high selectivity between exposed and unexposed regions of the photoresist to maintain critical dimensions, high anisotropy to give vertical profiles in the patterned photoresist and should also result in no residues after etching.
A major problem with known TSI resist processes is that RIE residue, in the form of "grass", is produced. RIE grass is a problem in both positive and negative working systems, since residue free images are desired. The grass is produced as a result of silicon being incorporated into regions to be etched, such that micromasks are formed in those regions, thus preventing the desired regions from being completely etched during etching, resulting in the "grass"-like residue.
It is known that the RIE grass problem can be eliminated by use of a two-step etch process for pattern development. The first step is a non-selective and aggressive process which removes silicon from the regions to be etched, either chemically using fluorine plasma or RIE, or physically using ion sputtering. The second step is a high selectivity oxygen etch to develop a grass free pattern. This two-step process is disadvantageous since it involves multiple process steps which introduce undesired effects, such as, process instability.
R. Lombaerts, B. Roland, A. Selino, A.M. Goethals, and L. Van den hove, Microelectron. Eng., 11 (1990), 543-547, discusses a method for optimizing an etch process for the DESIRE top surface imaging process using a two step etch process. The first step comprises a C.sub.2 F.sub.6 /O.sub.2 step to remove unwanted silicon from areas to be etched. The second step comprises a pure oxygen low power etch for pattern transfer. An alternate etch process included a two step oxygen only RIE process to obtain grass free results. The first step comprises a high power etch with low selectivity to remove unwanted silicon and further having a greater sputtering component in the etching process. The second step comprises a low power etch with high selectivity for grass free pattern transfer.
R.S. Hutton, R.L. Kostelak, O. Nalamasu, A. Kornblit, S. McNevin, and G.N. Taylor, "Application of Plasmask Resist and the DESIRE Process to Lithography at 248 nm", J. Vac. Sci. Technol. B, Microelectron. Process Phenom. (USA), Vol. 8, No. 6, Nov. Dec. 1990, p. 1502-8, discloses a method in developing a grass free RIE etch for the DESIRE process, employes a two-step etch wherein the first step comprises Ar.sup.+ ion sputtering to remove unwanted silicon prior to the second oxygen RIE step to obtain grass free images. In this process, significant amounts of resist are sputtered away prior to the second O.sub.2 RIE in order to minimize grass formation. This however results in patterns with considerable edge roughness.
The above discussed processes, however, are disadvantageous in the manufacturing environment. In particular, process stability is a major concern when using two-step fluorine containing etches. Residual fluorine in the O.sub.2 RIE step can have a catastrophic effect on line-width control during pattern development which results from deposition of varying amounts of carbon fluorine C,F polymer on the walls of the RIE chamber. This material then keeps on being released in the etching ambient with attendant variation in the process, resulting in an unstable process. That is, the chamber conditions change with each subsequent wafer.
Another disadvantage in using multi-chamber processes to improve stability is that it slows manufacturing throughput and increases total process time. Additionally, two step processes using sputtering to eliminate grass formation rely on a high selectivity O.sub.2 RIE which has a low etch rate. This low etch rate results in longer process times and a loss in throughput.
Thus it would be desirable to provide a process for top surface imaging to overcome the above identified problems and disadvantages. In particular, it would be desirable to provide a top surface imaging pattern transfer process for the fabrication of sub-micron feature size devices which is simple, provides high etch rates, and provides residue free images.